Computer Science 425 Distributed Systems CS 425 CSE 424 ECE 428

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Computer Science 425Distributed SystemsCS 425
/ CSE 424 / ECE 428
Indranil Gupta (Indy) August 23, 2007
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Our Only Goal Today

• To Define the Term Distributed System

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Can you name some examples of Operating Systems?
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Can you name some examples of Operating Systems?

• Linux WinXP Unix FreeBSD Mac
• 2K Aegis Scout Hydra Mach SPIN
• OS/2 Express Flux Hope Spring
• AntaresOS EOS LOS SQOS LittleOS TINOS
• PalmOS WinCE TinyOS

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What is an Operating System?
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What is an Operating System?

• User interface to hardware (device driver)
• Provides abstractions (processes, file system)
• Resource manager (scheduler)
• Means of communication (networking)

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FOLDOC definition
(FOLDOC Free On-Line Dictionary of Computing)

• The low-level software which handles the
  interface to peripheral hardware, schedules
  tasks, allocates storage, and presents a default
  interface to the user when no application program
  is running.
• The OS may be split into a kernel which is always
  present and various system programs which use
  facilities provided by the kernel to perform
  higher-level house-keeping tasks, often acting as
  servers in a client-server relationship.
• Some would include a graphical user interface and
  window system as part of the OS, others would
  not. The operating system loader, BIOS, or other
  firmware required at boot time or when installing
  the operating system would generally not be
  considered part of the operating system, though
  this distinction is unclear in the case of a
  roamable operating system such as RISC OS.
• The facilities an operating system provides and
  its general design philosophy exert an extremely
  strong influence on programming style and on the
  technical cultures that grow up around the
  machines on which it runs.

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Can you name some examples of Distributed
Systems?
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Can you name some examples of Distributed
Systems?

• Client-Server
• The Web
• The Internet
• An ad-hoc network
• A sensor network
• DNS
• Kazaa (peer to peer overlays)
• (The Solar System?)
• (Society?)
• (Food Chain?)

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What is a Distributed System?
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FOLDOC definition

• A collection of (probably heterogeneous)
  automata whose distribution is transparent to the
  user so that the system appears as one local
  machine. This is in contrast to a network, where
  the user is aware that there are several
  machines, and their location, storage
  replication, load balancing and functionality is
  not transparent. Distributed systems usually use
  some kind of client-server organization.

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Textbook definitions

• A distributed system is a collection of
  independent computers that appear to the users of
  the system as a single computer
• Andrew Tanenbaum
• A distributed system is several computers doing
  something together. Thus, a distributed system
  has three primary characteristics multiple
  computers, interconnections, and shared state
• Michael Schroeder

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Unsatisfactory

• Why are these definitions short?
• Why do these definitions look inadequate to us?
• Because we are interested in the insides of a
  distributed system
• design and implementation
• maintenance
• study
• Algorithmics (protocols)

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• I shall not today attempt further to define the
  kinds of material I understand to be embraced
  within that shorthand description and perhaps I
  could never succeed in intelligibly doing so. But
  I know it when I see it, and the motion picture
  involved in this case is not that.
• Potter Stewart, Associate Justice, US Supreme
  Court (talking about his interpretation of a
  technical term laid down in the law, case
  Jacobellis versus Ohio 1964)

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Which is a Distributed System (A) or (B)?
(A)
(A) Plants and Animals interacting in the Food
Chain
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(B)
(B) The Internet (Internet Mapping Project, color
coded by ISPs)
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A working definition for us

• A distributed system is a collection of
  entities, each of which is autonomous,
  programmable, asynchronous and failure-prone, and
  which communicate through an unreliable
  communication medium.
• Our interest in distributed systems involves
• design and implementation, maintenance, study,
  algorithmics
• Entitya process on a device (PC, PDA)
• Communication MediumWired or wireless network

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A range of interesting problems for Distributed
System designers

• Basic Concepts Asynchrony,Consensus,
• Routing IP,BGP
• Large-scale Systems The Grid,Gnutella,Kazaa
• Distributed File Systems NFS,AFS
• Protocols, e.g., multicast IP multicast, SRM,
  RMTP
• CoordinationSETI_at_Home,Multiplayer online games
• Storage and Databases
• Security

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Distributed Systems Design Goals

• Common Goals
• Heterogeneity can the system handle different
  types of PCs and devices?
• Robustness is the system resilient to host
  crashes and failures, and to the network dropping
  messages?
• Availability are data, services always there
  for clients?
• Transparency can the system hide its internal
  workings from the users?
• Concurrency can the server handle multiple
  clients simultaneously?
• Efficiency is it fast enough?
• Scalability can it handle 100 million nodes?
  (nodesclients and/or servers)
• Security can the system withstand hacker
  attacks?
• Openness is the system extensible?

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Distributed System Example -- the Internet
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The Internet

• A vast interconnected collection of computer
  networks of many types.
• Intranets subnetworks operated by companies and
  organizations.
• ISPs companies that provide modem links and
  other types of connections to users.
• Intranets are linked by backbones network links
  of large bandwidth, such as satellite
  connections, fiber optic cables, and other
  high-bandwidth circuits.

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A Typical Intranet
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Intranets

• Composed of several local area networks (LANs)
  linked by backbone connections.
• Connected to the Internet via a router/multiple
  routers.
• A firewall is used to protect an intranet (from
  the outside Internet) by preventing unauthorized
  message leaving/entering, and is implemented by
  filtering incoming and outgoing messages.

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Internet Apps Their Protocols and Transport
Protocols
Application layer protocol smtp RFC 821 telnet
RFC 854 http RFC 2068 ftp RFC
959 proprietary (e.g. RealNetworks) NFS proprieta
ry (e.g., Skype)
Underlying transport protocol TCP TCP TCP TCP TCP
or UDP TCP or UDP typically UDP
Application e-mail remote terminal access Web
file transfer streaming multimedia remote file
server Internet telephony
Implemented via network sockets. Basic
primitive that allows machines to send messages
to each other
TCPTransmission Control Protocol UDPUser
Datagram Protocol
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WWW the HTTP Protocol

• HTTP hypertext transfer protocol
• WWWs application layer protocol
• client/server model
• client browser that requests, receives, and
  displays WWW objects
• server WWW server stores the website, and sends
  objects in response to requests
• http1.0 RFC 1945
• http1.1 RFC 2068

http request
PC running Explorer
http response
http request
Server Running cnn.com Web server
http response
Mac running Navigator
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The HTTP Protocol More

• http TCP transport service
• client initiates a TCP connection (creates
  socket) to server, port 80
• server accepts the TCP connection from client
• http messages (application-layer protocol
  messages) exchanged between browser (http client)
  and WWW server (http server)
• TCP connection closed

• http is stateless
• server maintains no information about past client
  requests

aside

• Protocols that maintain state are complex!
• past history (state) must be maintained
• if server/client crashes, their views of state
  may be inconsistent, and hence must be
  reconciled.

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HTTP Example

• Suppose user enters URL www.cs.uiuc.edu/

(contains text, references to 10 jpeg images)

• 1a. http client initiates a TCP connection to
  http server (process) at www.cs.uiuc.edu. Port 80
  is default for http server.

1b. http server at host www.cs.uiuc.edu waiting
for a TCP connection at port 80. accepts
connection, notifying client
2. http client sends a http request message
(containing URL) into TCP connection socket
3. http server receives request messages, forms a
response message containing requested object
(index.html), sends message into socket
time
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HTTP Example (cont.)
4. http server closes the TCP connection.

• 5. http client receives a response message
  containing html file, displays html, Parses
  html file, finds 10 referenced jpeg objects

6. Steps 1-5 are then repeated for each of 10
jpeg objects
time

• For fetching referenced objects, have 2 options
• non-persistent connection only one object
  fetched per TCP connection
• some browsers create multiple TCP connections
  simultaneously - one per object
• persistent connection multiple objects
  transferred within one TCP connection

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Trying Out HTTP (Client Side) for Yourself

• 1. Telnet to your favorite WWW server

Opens TCP connection to port 80 (default http
server port) at www.cnn.com Anything typed in
sent to port 80 at www.cnn.com
telnet www.cnn.com 80
2. Type in a GET http request
By typing this in (hit carriage return twice),
you send this minimal (but complete) GET request
to http server
GET /index.html HTTP/1.0
3. Look at response message sent by http server!
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Does our Working Definition work for the http
Web?

• A distributed system is a collection of
  entities, each of which is autonomous,
  programmable, asynchronous and failure-prone, and
  that communicate through an unreliable
  communication medium.
• Our interest in distributed systems involves
• design and implementation, maintenance, study,
  algorithmics
• Entitya process on a device (PC, PDA)
• Communication MediumWired or wireless network

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Important Distributed Systems Issues

• No global clock no single global notion of the
  correct time (asynchrony)
• Unpredictable failures of components lack of
  response may be due to either failure of a
  network component, network path being down, or a
  computer crash (failure-prone, unreliable)
• Highly variable bandwidth from 16Kbps to Tbps
• Possibly large and variable latency few ms to
  several seconds
• Large numbers of hosts 2 to several million

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Important Issues

• If youre already complaining that the list of
  topics weve discussed so far has been
  perplexing
• Youre right!
• It was meant to be (perplexing)
• The Goal for the Rest of the Course see enough
  examples and learn enough concepts so these
  topics and issues will make sense
• We will revisit many of these slides in the very
  last lecture of the course!

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Concepts?

• Which of the following inventions do you think is
  the most important?
• The PDA
• The PC
• The transistor
• Which of the following inventions do you think is
  the most important?
• Email
• The Web
• TCP/IP
• What lies beneath? Concepts!

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How will you Learn?

• Take a look at handout Course Information and
  Schedule
• Text Colouris, Dollimore and Kindberg (4th
  edition)
• Lectures
• Homeworks
• Approx. one every two weeks
• Solutions need to be typed, figures can be
  hand-drawn
• Programming assignments
• Incremental, in 3-4 stages
• We will build a peer to peer system!
• Exams/quizzes
• Midterm, and final

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On the Textbook

• Text Colouris, Dollimore and Kindberg (4th
  edition). White book.
• The 3rd edition will suffice for most material
  too. However, we will refer to section, chapter,
  and problem numbers only in the 4th edition.
• The 3rd edition may have a different numbering
  for some HW problems (that we give from the
  textbook). Make sure you solve the right problem
  the responsibility is yours (no points for
  solving the wrong problem!)

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What assistance is available to you?

• Lectures
• lecture slides will be placed online at course
  website
• Tentative version before lecture
• Final version after lecture
• Homeworks office hours to help you (without
  giving you the solution)
• Programming Assignments (MPs) program templates
  will be given to you. C (or C) programming.
• An additional assignment may be given that is
  open-ended, and can turn into a research project.
  
• Course Prerequisite Operating Systems/Systems
  Programming (CS 241 or CS 423 or instructor
  permission)

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If youre still thinking, Everything youve
said so far is so boring...

• CS425 is about enjoying distributed systems,
  learning a few new things, and designing some
  cool new systems that you can boast about to your
  friends (and job interviewers).
• Were here to help you achieve all these things.

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You can meet us anytime

• In person (Office Hours)
• Myself (Indy) 3112 Siebel Center
• Every Tuesday 3.30 PM to 5 PM, and Every Thursday
  2 PM to 3.30 PM
• TA, Ramses Morales 207 Siebel Center
• Every Monday 10.30 AM to 12 PM, and Every
  Wednesday 12.30 PM to 2 PM
• Virtually
• Newsgroup class.cs425 (most preferable,
  monitored daily) 24 hour turnaround time for
  questions!
• Email (turnaround time may be longer than
  newsgroup)

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Readings

• For todays lecture Chapter 1
• For next Tuesdays lecture
• Read sections 11.1-11.4
• Fill out and return Student InfoSheet

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Have a Good Weekend!